1. Field
This disclosure relates generally to the field of artificial intervertebral discs, and more particularly to an implantable polymeric replacement disc.
2. Description of Related Art
Artificial intervertebral (or spinal) discs are gaining acclaim as a treatment for degenerative disc disease and as a viable alternative to spinal fusion surgery. Substantial research, testing and prototyping has occurred over recent decades, in an effort to develop a replacement disc which is easy to implant and adequately retained in an appropriate orientation once placed, and which imitates the properties and functions of a natural disc. Development efforts have also focused on creating a replacement disc that is flexible (e.g., imitable of the 2-3% flexion of natural discs), accommodating of loading and movement, and which does not impinge or otherwise adversely affect the adjacent vertebrae or spinal nerve roots. Likewise, much consideration has been given to durability and biocompatibility of artificial disc materials, as the artificial disc should ideally be durable enough to function throughout the life of the recipient, without rejection.
Accordingly, prior art artificial discs run the gambit, from rigid implantable devices retained with screws in adjacent vertebrae, to simple sponge-like discs positioned and retained between vertebrae. U.S. Patent Application Publication No. 2005/0055099, by Ku, describes one such artificial disc, which is porous to facilitate tissue ingrowth, both for additional stabilization of the disc and with the aim of allowing the disc, over time, to more closely imitate the functioning of a natural spinal disc. Other solutions combine inflexible and resilient components, for example sandwiching the resilient material between plates made from metal or another similarly rigid material. Often, the rigid plates have external protrusions or spines which are intended to seat into adjacent vertebrae. These sandwich type replacement discs are commonly screwed into adjacent vertebrae or else retained via complex retention brackets, wires, ties, bolts, rivets and the like. Implanting these “fixed” replacement discs thus requires the surgeon to drill into adjacent vertebrae or to use a variety of hardware to otherwise fix the replacement disc to the adjacent vertebrae.
These conventional manners of fixing artificial replacement discs may not only damage the adjacent vertebrae, but also limit spinal flexibility, excessively compress the replacement disc, adversely affect replacement disc durability, increase recipient recovery time and increase the risk of disc rejection by the recipient. Each of these adverse effects may compromise overall health of the recipient, necessitating additional medical treatment and even surgery.
Consequently, there has been movement away from artificial discs which require substantial hardware (e.g., screws, brackets or wires) to mount in place. The early 1990s to present has been marked by a trend toward greater resilience, less rigidity and less surgical attachment to adjacent vertebrae. For example, the aforementioned Ku application and also U.S. Patent Application Publication No. 2005/171611, by Stoy, disclose flexible discs.
The Stoy application particularly discusses a prosthetic disc having parallel layers of hydrogel. The rigidity of the disc is determined by the number of parallel layers fabricated. However, similar to previously discussed replacement discs, the Stoy application discusses the need for at least one rigid layer. It is also complex to manufacture and somewhat cumbersome to implant, because the disc is dehydrated during implantation and then must be hydrated to achieve a desired stiffness to flexibility ratio. A similar artificial disc is disclosed in U.S. Pat. No. 5,458,543, issued to Oka.
The Ku application moves away from the trend of layered artificial discs, disclosing a unitary elastomer which is manufactured with specific chemical properties to substantially replicate the nucleus of a natural spinal disc. However, the Ku disc must be mechanically fixed in place. It is thus provided with a protrusion or appendage extending outward from the flexible portion of the disc, for mechanically fixing to the adjacent vertebrae.